This invention relates to a photovoltaic device which converts light to electricity. In particular, this invention relates to a photovoltaic device which can have a front surface with an attractive appearance.
A wrist watch which uses a photovoltaic device as its power source has been developed. Since this type of watch is operated by electric power from a photovoltaic device, battery replacement is unnecessary.
For example, a prior art configuration is disclosed in Japanese Non-examined Patent Publication No.8-211162 which issued on Aug. 20, 1996. In this publication, a photovoltaic structure is disclosed comprising a photovoltaic device with a substrate having an insulating surface, a metal electrode layer, an amorphous silicon layer, and a transparent electrode layer, and a transparent electrode layer on top of the photovoltaic element. Further, the disclosed structure has a light diffuse transmission filter disposed on top of the transparent resin layer. Finally, the photovoltaic structure with the light diffuse transmission filter layered on top is assembled into a wrist watch housing.
Another prior art structure is disclosed in Japanese Non-examined Patent Publication No.8-5752 which issued on Jan. 12, 1996. In this patent application, the photovoltaic structure is cited only as an amorphous silicon layer formed on a substrate. Although details are not cited, it is believed to have the same photovoltaic structure as the previously mentioned disclosure, with a transparent resin layer on the light receiving surface. Further, a light diffuse transmission filter is disposed via spacers above the photovoltaic structure and the unit is assembled into a wrist watch housing.
A photovoltaic device which is disposed on an often viewed component such as the dial of a watch, greatly affects the design of that watch. A photovoltaic device fixed to a watch dial, does not always result in a watch dial having optimum coloration or appearance. The surface of a photovoltaic device is designed with the objective of efficiently absorbing light energy and producing as much electrical power as possible. The surface of a photovoltaic device is a dark color reflecting the color of the active photovoltaic semiconductor layer and interference coloration from the light receiving transparent electrode layer. A watch dial with an attractive appearance cannot always be designed with a dark colored photovoltaic device. Watch dials are typically colored with bright colors, such as white, rather than dark colors.
Further, a photovoltaic device is divided into a plurality of sections connected in series to increase the output voltage. The pattern of a photovoltaic device, divided into a plurality of sections, is perceptible at its surface. This type of pattern also does not necessarily provide a desirable decorative watch dial. For these reasons, a light diffuse transmission filter is disposed over the entire surface of the prior art photovoltaic structures.
However, in the prior art photovoltaic structures mentioned above, a layer of transparent resin is formed. A photovoltaic element with a transparent resin layer has an output characteristic which is only about 90% that of a photovoltaic element with no transparent resin layer. Further, when a light diffuse transmission filter is disposed over the entire surface of a photovoltaic structure having a transparent resin layer, the output characteristic drops to about 42% that of a photovoltaic element with no transparent resin layer after transparent electrode formation.
In this type of photovoltaic device, a light diffuse transmission filter is provided by applying transparent resin with particles mixed into the surface of the photovoltaic element. This light diffuse transmission filter can make a dark colored structure appear white. A photovoltaic device having transparent resin containing particles applied to the surface of the photovoltaic element is disclosed in Japanese Utility Model Publication No.5-36284 which issued on Sep. 14, 1993.
However, a photovoltaic device of this structure has the drawback of a detrimental effect on the photovoltaic element when transparent resin containing particles are applied. For example, if transparent resin containing particles are applied to the surface of the photovoltaic element by screen printing, the particles added to the transparent resin pierce into the amorphous silicon layer and short circuit the transparent electrode and metal electrode layers.
The present invention was developed to solve these types of problems.
Thus, it is a primary object of the present invention to provide a photovoltaic device which does not incur damage to the photovoltaic element, and has a surface which can be formed with an attractive color and design.